Xamarin iOS UIAlertController 未显示

Question

在我的程序中，当您点击第一个视图控制器上的 'begin' 按钮时，它会将您带到一个名为 VCLoadLocalData 的新视图控制器。当该视图控制器加载时，它将检查某个 txt 文件是否存在，如果存在，它将显示一个需要用户输入的警报，如果不存在，它将显示另一个警报。目前，UIAlertController 没有出现；我已经用 UIAlertView 试过了，但我不知道如何让用户响应 return 或让它触发一个方法。这是我目前拥有的 VCLoadLocalData 代码：

using Foundation;
using System;
using System.IO;
using System.Threading.Tasks;
using UIKit;

namespace SixthFormFinder
{

    public partial class VCLoadLocalData : UIViewController
    {

        public VCLoadLocalData (IntPtr handle) : base (handle)
        {

        }

        public override void ViewDidLoad()
        {
            base.ViewDidLoad();
            // Perform any additional setup after loading the view, typically from a nib.
            bool doesExist = File.Exists("UserData.txt");
            if (doesExist)
            {
                //UIAlertView alert = new UIAlertView()
                //{
                //    Title = "User Data Found",
                //    Message = "A version of your data has been found, do you want to use it?"
                //};
                //alert.AddButton("Yes");
                //alert.AddButton("No");
                //alert.Show();
                var alert = UIAlertController.Create("User Data Found", "A version of your data has been found, do you want to use it?", UIAlertControllerStyle.Alert);
                alert.AddAction(UIAlertAction.Create("Yes", UIAlertActionStyle.Default, (UIAlertAction obj) => Alert_Yes()));
                alert.AddAction(UIAlertAction.Create("No", UIAlertActionStyle.Default, (UIAlertAction obj) => Alert_No()));
                ShowViewController(alert, null);
            }
            else
            {

                var alert = UIAlertController.Create("No User Data", "No version of your user data was found, you will need to follow this setup", UIAlertControllerStyle.Alert);
                alert.AddAction(UIAlertAction.Create("OK", UIAlertActionStyle.Default, (UIAlertAction obj) => Alert_No()));
                ShowViewController(alert, null);
            }
        }

        void Alert_Yes()
        {
            var alert = UIAlertController.Create("Yes", "Yes Test complete", UIAlertControllerStyle.Alert);
            ShowViewController(alert, null);
        }

        void Alert_No()
        {
            var alert = UIAlertController.Create("No", "No Test complete", UIAlertControllerStyle.Alert);
            ShowViewController(alert, null);
        }
    }
}

Answer 1

在方法ViewDidLoad中，视图不是initialized.You可以将你的代码移动到方法ViewDidAppear中。

public override void ViewDidAppear(bool animated)
{
  base.ViewDidAppear(animated);

  bool doesExist = File.Exists("UserData.txt");
  if (doesExist)
   {
     var alert = UIAlertController.Create("User Data Found", "A version of your data has been found, do you want to use it?", UIAlertControllerStyle.Alert);
     alert.AddAction(UIAlertAction.Create("Yes", UIAlertActionStyle.Default, (UIAlertAction obj) => Alert_Yes()));
     alert.AddAction(UIAlertAction.Create("No", UIAlertActionStyle.Default, (UIAlertAction obj) => Alert_No()));
     ShowViewController(alert, null);
   }
  else
   {

     var alert = UIAlertController.Create("No User Data", "No version of your user data was found, you will need to follow this setup", UIAlertControllerStyle.Alert);
     alert.AddAction(UIAlertAction.Create("OK", UIAlertActionStyle.Default, (UIAlertAction obj) => Alert_No()));
     ShowViewController(alert, null);
    }
}

请注意，这段代码可能会执行多次，每当 VCLoadLocalData appears.So 如果您只是检查一次 time.You 可以改进代码。

 . . .
 private bool isFirstLoad = true;
 . . .
 public override void ViewDidAppear(bool animated)
 {
   base.ViewDidAppear(animated);

   if(isFirstLoad)
   {
    isFirstLoad=false;
    bool doesExist = File.Exists("UserData.txt");
    if (doesExist)
     {
      var alert = UIAlertController.Create("User Data Found", "A version of your data has been found, do you want to use it?", UIAlertControllerStyle.Alert);
      alert.AddAction(UIAlertAction.Create("Yes", UIAlertActionStyle.Default, (UIAlertAction obj) => Alert_Yes()));
      alert.AddAction(UIAlertAction.Create("No", UIAlertActionStyle.Default, (UIAlertAction obj) => Alert_No()));
      ShowViewController(alert, null);
     }
    else
     {

      var alert = UIAlertController.Create("No User Data", "No version of your user data was found, you will need to follow this setup", UIAlertControllerStyle.Alert);
      alert.AddAction(UIAlertAction.Create("OK", UIAlertActionStyle.Default, (UIAlertAction obj) => Alert_No()));
      ShowViewController(alert, null);
     }
   }

}

Xamarin iOS UIAlertController 未显示

Xamarin iOS UIAlertController not showing

c#

uiviewcontroller

xamarin.ios

xamarin

xamarin.forms